Known engine fuel injection systems include accumulators for minimizing pressure variations in the fuel pressure at the injectors. Some fuel injection systems include controls for varying the fuel pressure. In such systems it is necessary that the accumulator pressure be varied in accordance with fuel pressure variations so that the accumulator can continue to perform its function as a source of immediately available energy. In known systems employing a closed gas-volume accumulator, the gas pressure is changed by adding or withdrawing a volume of fuel sufficient to displace a piston or diaphragm to compress or expand the gas. As a result, not only does gas volume change, altering the characteristics of the accumulator, but, if rapid pressure changes are involved, the fuel required to compress the gas must be supplied at a high flow rate requiring that the supply pump respond to the instantaneous demand rather than average demand, thus to some extent negating the storage advantage of the accumulator.
My invention provides a means for adding or withdrawing gas to the accumulator in proportion to the changes in fuel pressure, thus holding the volume of fuel in the accumulator constant and avoiding additional burden on the supply pump. It includes a floatable valve element for alternately communicating the accumulator with a source of high gas pressure or an atmospheric vent, to raise or lower the accumulator pressure in accordance with step changes in fuel pressure. The valve is operated by pressure signals from the fuel system and accumulator.